This invention relates to separating a first salt and a second salt from an aqueous solution containing both salts, the solubility of the first salt increasing more with increasing temperature than the solubility of the second salt. More particularly this invention relates to separating these salts from an aqueous solution containing those salts and salt impurities by passing the solution through a series of evaporators.
Salts having a solubility increasing with increasing temperatures within a given temperature range, e.g., potassium chloride, hereinafter called the first salts, and salts having a solubility relatively unchanged or decreasing with increasing temperatures within that same temperature range, e.g., sodium chloride, hereinafter called second salts, are frequently found in mixtures in naturally occurring ores. In a recognized method of producing the salts, these ores are dissolved in a suitable aqueous solvent, thereby forming a solution from which the salts can be easily separated. Admixtures of these salts in solution may also arise during the course of some industrial chemical production, e.g., electrolysis of electrolytes.
The first and second salts are separated by concentrating the solution by evaporation until the "invariant composition" is approached or reached. By "invariant composition" is meant a composition at which a solution at a given temperature is saturated with two or more salts. Thus, for an invariant composition in the instant case the solution is saturated with respect to the first and second salt. The evaporation is carried out at such a temperature that the second salt is selectively precipitated (owing to the relative solubilities of the salts) and removed as the invariant composition is approached. Accordingly, evaporation at incrementally higher temperatures can deplete the solution of the second salt while concentrating the solution with respect to the first salt without precipitating the first salt. The second salt depleted solution is then forwarded to a first salt recovery step where the first salt is removed from the solution, e.g., by incrementally cooling the solution to selectively precipitate the first salt (again, owing to the relative solubilities of the salts).
Naturally occurring brines, solution mined ore, or dissolved shaft or room and pillar mined ore containing potassium chloride, for example, can be treated by the above described process for the recovery of potassium chloride (first salt) principally and sodium chloride (second salt) secondarily. Usually, these potassium chloride-containing ores contain significant amounts of impurities such as magnesium chloride, calcium carbonate, magnesium sulfate, calcium chloride, calcium sulfate, and sodium sulfate. Significant amounts of impurities, e.g., sodium sulfate and calcium sulfate, are precipitated along with sodium chloride during the evaporation step. Also, the presence of a few parts per hundred parts of water of an impurity, such as magnesium chloride will lower the invariant composition of a potassium chloride-sodium chloride solution by a few parts each of potassium chloride and sodium chloride per hundred parts of water, thereby lowering the temperature at which the invariant composition is reached during the evaporation step.
In an industrial method, separation of potassium chloride and sodium chloride from an aqueous solution containing both salts is effected by multiple effect evaporation operated at progressively higher temperatures in the direction of the flow of the solution, often described as backward feed; that is, mother liquor effluent overflow from cooler effects is forwarded to hotter effects. A typical evaporator effect for purposes of concentrating salts, comprises an evaporator communicating with an elutriation leg. As the solution passes through each effect, water is removed and the solution becomes concentrated with respect to potassium chloride while precipitating some salt impurities and sodium chloride which settles into the elutriation leg. Sodium chloride is precipitated in each evaporator effect until the solution approaches or reaches its invariant composition for the temperature at which each evaporator effect is operated.
These evaporator effects are heated in a direction opposite to the direction of the flow of the solution to be concentrated. The first effect is heated by introducing steam from an external source, such as a boiler, and successively higher effects are heated by the vapors from the preceding effects progressively to the last evaporator effect wherefrom vapor is forwarded to a cooling tower. By operating the evaporators in this multiple effect manner, greater efficiency is achieved through more product recovery and greater steam economy.
The first evaporator effect mother liquor effluent overflow, which is concentrated with respect to potassium chloride, is forwarded to a potassium chloride recovery step. The recovery step is commonly a series of crystallizers in which the first effect mother liquor effluent is cooled to precipitate potassium chloride. Due to the aforedescribed solubility characteristic of sodium chloride, the potassium chloride crystallized is recovered nearly free of precipitated sodium chloride.
The effluent from the potassium chloride recovery step may be depleted to low potassium chloride levels, e.g., by refrigerative crystallization methods. The potassium chloride content of the effluent from the potassium chloride recovery step can be depleted to about 8 parts potassium chloride per 100 parts water, or lower. Part of this effluent can be purged before reintroduction into the evaporators to avoid increases in salt impurity levels above an undesirable amount, e.g., greater than about 4 percent salt impurities by weight.
As stated above, significant amounts of impurities precipitate along with sodium chloride. These precipitated impurities settle to the bottom of the evaporator and into the elutriation leg in communication therewith. In the above described evaporation, sodium chloride crystals are larger (faster settling rate crystals) than essentially all of the crystals of the salt impurities. Therefore, most of the impurities entering the elutriation leg may be fluidized by introducing a liquid at the lower portion of the elutriation leg to effect a net liquid upward flow countercurrent to the settling solids (often described as elutriating). The rate at which the elutriating liquid rises is such that substantially all of the fine particle impurities are carried back up into the evaporator and forwarded to subsequent evaporators along with the mother liquor effluent overflow forwarded thereto, while sodium chloride crystals are allowed to settle to the bottom of the elutriation leg for removal therefrom as relatively pure crystals.
The mother liquor effluent overflow from the first effect evaporator, which contains a significant amount of fluidized impurities, may be forwarded to a zone where the impurities are removed, such as in a settling zone operated at quiescent conditions. Upon the addition of floculating agents to the settling zone, the floculated settled particles easily removed. Then, the mother liquor therefrom is forwarded to the potassium chloride recovery step.
As stated above, the impurities settling in the elutriation legs are fluidized by a liquid introduced near or at the bottom of the elutriation legs. The liquids so utilized are the raw feed solution or the mother liquor effluent from the potassium chloride recovery step. The raw feed solution is used to fluidize the cooler evaporator effects while the mother liquor effluent from the potassium chloride recovery step is used to fluidize the hotter evaporator effects, thereby providing fluidizing liquids at or near the operating temperature of the evaporator effect being fluidized (See, for example, U.S. Pat. No. 3,365,278 and U.S. Pat. No. 3,704,101). The temperature of these fluidizing streams may be adjusted by some heat exchange mechanism before the fluidizing liquid is introduced into the evaporator effects, thereby improving steam economy. However, not only is it desired that the fluidizing liquid be of a compatible temperature with the liquid in the evaporator effect, but it is also desired that the fluidizing liquid be of a compatible salt composition with the solution in the evaporator effect. However, neither the raw feed solution nor first salt recovery step mother liquor effluent satisfies this compatibility composition requirements for all the evaporator effects. It is, therefore, a desideratum that a liquid with temperature and salt composition compatibility be available as an elutriant.